In recent years there is an increasing need for printers able to print faster and with a higher print density. Laser printers, which employ a laser light source, and LED (light-emitting diode) printers, which employ an LED array as the light source, are two examples of printers used in response to such needs. While a laser printer requires the use of a mechanical mechanism, such as a rotating polygonal mirror, for the scanning laser beam, with an LED printer it is only necessary for the light-emitting diodes (hereinafter also referred to as "light-emitting elements") that make up the light-emitting diode array to be electrically controlled. The advantages of LED printers compared with laser printers are that as they do not have any mechanical moving parts, they are smaller, faster and more reliable.
Referring to FIG. 1, there is shown a cross-sectional view of two light-emitting elements 8 of a prior art AlGaAs-based homojunction LED array 9. The array 9 comprises a substrate 10 of p-type conductivity GaAs having on a surface 11 thereof a first layer 12 of p-type conductivity Al.sub.x Ga.sub.1-x As. The first layer 12 is about 10 microns in thickness and is doped with Zn to a concentration of about 5.times.10.sup.17 impurities/cm.sup.3. On a surface 24 of the first layer 12 is a second layer 14 of n-type conductivity Al.sub.y Ga.sub.1-y As which is about 5 microns in thickness and is doped with Te to a concentration of about 8.times.10.sup.17 impurities/cm.sup.3. For emitting light with a wavelength in the region of 720 nm., the aluminum composition in the first and second layers 12 and 14 is set at x=0.2 and y=0.5. Spaced grooves 15 extend through the second layer 14 and a portion of the first layer 12 to form mesa like light-emitting elements 8.
A separate contact layer 16 of n+ type conductivity GaAs is on a portion of the second layer 14 of each light-emitting element 8. The contact layers 16 are about 0.1 microns in thickness and are doped with Sn to a concentration of about 5.times.10.sup.18 impurities/cm.sup.3. On each of the contact layer 16 is a separate electrode 18, and on a surface 13 of the substrate 10 is an electrode 20. An antireflection coating 22 of the SiN.sub.x covers the light-emitting elements 8 and the bottom of the grooves 15.
The array 9 is made by depositing on the surface 11 of the substrate 10 in succession using liquid-phase epitaxy (LPE), the first layer 12, the second layer 14 and a contact layer 16. The electrode layer 18 is deposited on the contact layer 16 and the electrode layer 20 is deposited on the surface 13 of the substrate 10. Using photolithography and plasma etching, the electrode layer 18 is defined to leave portions of the electrode layer 18 only over the area which is to form the light-emitting elements 8. Then using a chemical etchant of NH.sub.4 OH:H.sub.2 O.sub.2 =1:10, the contact layer 16 is removed except for the portions under the elecrodes 18. Using photolithography and a chemical etchant of H.sub.2 SO.sub.4 :H.sub.2 O.sub.2 :H.sub.2 O=1:2:40, the grooves 15 are etched through the second layer 14 and about one micron into the first layer to form the light-emitting elements 8. Plasma CVD (chemical vapor deposition) is then used to form the antireflection SiN.sub.x coating 22. This is followed by alloying to form the electrodes 18 and 20.
Structurally, this heterojunction light-emitting diode array 9 consists of discrete high-luminance LEDs 8 arranged into a single array. Using the n-type conductivity Al.sub.y Ga.sub.1-y As second layer 14 that is transparent to the light emitted by the emission layer p-type conductivity Al.sub.x Ga.sub.1-x As first layer 12 results in energy attenuation from internal absorption being avoided. Also, an epitaxial junction with excellent crystallinity is employed, and the improvement in injection efficiency provided by the heterojunction raises the overall external emission efficiency.
However, unlike the case with single, discrete LEDs, there are the following problems with LED arrays used for printer applications. First, optical crosstalk between elements has to be suppressed; and second, variation in characteristics from element to element has to be minimized.
For this, in the light-emitting diode array 9 shown in FIG. 1, the n-type Al.sub.y Ga.sub.1-y As second layer 14 that forms a transparent window has to be completely removed between light-emitting elements 8. In addition, to reduce optical bleeding, the emission mesas have to be formed to a certain minimum depth into the p-type Al.sub.x Ga.sub.1-x As emission first layer 12.
The diffusion length of minority carrier electrons injected into the p-type Al.sub.x Ga.sub.1-x As first layer 12 decreases as the distance from the p-n junction increases, but is in the order of 10 microns. This means that at least about 10 microns of the p-type Al.sub.x Ga.sub.1-x As first layer 12 has to be removed. However, it is difficult to accomplish this with adequate process uniformity and reproducibility. Thus, some degree of optical bleeding has been unavoidable in the resultant LED arrays in which the fabrication process took such factors into account. Furthermore, the p-type Al.sub.x Ga.sub.1-x As first layer 12 that is within the diffusion length of electrons from the p-n junction 24 functions effectively as an emission layer. Thus, in order to optimize the emission efficiency, it is necessary to make the p-type Al.sub.x Ga.sub.1-x As first layer 12 at least 10 microns thick. A problem is, however, that even if the emission efficiency is improved, owing to the high refractive index of the light-emitting portion, most of the light is lost through total reflection. This results in a very low external light output efficiency of no more than several percent. One cause may be the loss of high-intensity light in the emission portion directly beneath the electrode 18, owing to the fact that the electrode is formed directly on the upper part of the emission portion.